waves in the ether. If we assume that heat-waves The are only long light-waves we may conclude that they travel with the same velocity as light. theory was perfectly well stated by Dr. Thomas Young, the only contemporary of Rumford and Davy who had clear ideas on the subject. He says: If heat be not a substance it must be a quality, and this quality can only be motion. It was Newton's opinion that heat consists in a minute vibratory motion of the particles of bodies, and that this motion is communicated through an apparent vacuum by the undulations of an elastic medium, which is also concerned in the phenomena of light. If the arguments which have been lately advanced in favour of the undulating nature of light be deemed valid, there will be still stronger reasons for admitting this doctrine respecting heat, and it will only be necessary to suppose the vibrations and undulations principally constituting it to be larger and stronger than those of light, while at the same time the smaller vibrations of light, and even the blackening rays, derived from still more minute vibrations, may perhaps, when sufficiently condensed, concur in producing the effects of heat. These effects, beginning from the blackening rays, which are invisible, are a little more perceptible in the violet, which still possess but a faint illuminating power; the yellow-green afford the most light; the red give less light, but much more heat; while the still larger and less frequent vibrations, which have no effect on the sense of sight, may be supposed to give rise to the least refrangible rays, and to constitute invisible heat." The "blackening rays" referred to by Young are rays of too short wave-length to affect our sense of sight, but which can be detected by their chemical effects. We now know that the range of "etherwaves is vastly greater than was suspected by Young, extending, as they do, from the electromagnetic waves used in wireless telegraphy to the recently discovered "cosmic rays," which are shorter than the shortest X-Rays. But we see that, even a century ago, the two great phenomena of light and heat were linked up, in certain aspects at least, in one great generalization. This generalization was still further generalized, and a great new region won for science, when Maxwell created his electromagnetic theory of light. CHAPTER V MOLECULES AND ATOMS We have said that the theory that matter is composed of atoms and molecules was first given precise form by Dalton, at the beginning of the nineteenth century. The hypothesis, as we have seen, was a very old one. It existed in a vague form amongst the ancient Greeks. But Dalton was the first man to put it in a form that admitted of quantitative verifications. The theory was a great success in chemistry from the very moment it was enunciated, but it is only in recent times that nobody can be found to say a word against it. Goethe was opposed to it, and we may also mention the more scientific names of Mach and Ostwald. Faraday doubted it. But the theory has now been confirmed with such precision, and in so many different ways, that doubt is no longer possible. It is true that the actual shape and constitution of the atom are matters that still present great difficulties, but as against the theory that matter is continuous the atomic theory has triumphed. A great deal can be done with the atomic theory even on the crude supposition that atoms are small hard spheres. As an illustration of this we may mention, besides its triumphs in elementary chemistry, two of its physical applications, the kinetic theory of gases, and the Brownian movement. If matter is composed of small separate particles we may suppose that, in a solid, the movements of these particles are severely restricted. The motions that constitute heat in a solid we may suppose to consist in small oscillations about a centre. Yet even here we have evidence of actual translatory motions of the molecules. With pairs of metals that have been kept in contact for years, we find that the bottom layer of the top metal and the top layer of the lower metal have become to some extent intermingled. With liquids and gases, of course, diffusion is much more rapid and complete. Thus, if a globe containing hydrogen, the lightest of gases, be placed above a globe containing carbon dioxide, a heavy gas, and the two globes be put in communication by a stopcock, it is found, after a short time, that each globe contains as much carbon dioxide as hydrogen. Liquid diffusion takes longer, but even if ether and water be superposed it is found, after a time, that every part of the layer of ether contains some water and that in every part of the water there is some ether. The maximum freedom of movement, however, is possessed by the molecules of a gas. On the molecular theory, as we see, we may suppose a gas to consist of an enormous number of particles moving in all directions at random. We may further suppose that some particles are moving more swiftly than others, and that the directions of the particles are continually changing, due to collisions. The kinetic theory of gases is the attempt, inaugurated by Maxwell, to deal mathematically with this chaotic assemblage. It would obviously be hopeless to attempt to follow the history of one particular molecule. Maxwell therefore introduced statistical methods and applied the theory of probabilities. The mere fact that the molecular chaos could be assumed to be completely chaotic was found to provide the key to the problem. In this investigation the molecules were supposed to obey the ordinary laws of mechanics, and on this basis, the well-known laws governing the behaviour of gases were successfully derived from the theory, and certain new phenomena predicted which were experimentally confirmed. There is no need, for our purposes, to pass these results in review, but we may refer to the very useful notion of "absolute temperature " which is made very clear by this theory. The temperature of a gas is a measure of the kinetic energy of its molecules, and the way in which this energy changes with the temperature was calculated. It changes in such a way that, at a temperature 273 degrees below zero on the centigrade scale, it becomes zero. This, therefore, is the absolute zero of temperature. No body can be colder, since energy cannot assume negative values. The actual velocities of the molecules of a gas, at ordinary temperatures, as derived from this theory are very considerable. Molecules of air are moving with about the velocity of a rifle bullet. On an |